my setup looks like that from OE9WLJ (POTY much closer to the mounting bracket than in your images) and I have only about 1dB "loss" compared to the LNB in the focus without the POTY. And like PA1EW already said the dielectric lens is not in the right postion.

I see two issues, as others observe the lens isn't fully inserted, and the feed is not at the focus. The latter is really important, a few cm error will have a big impact at 10 GHz. You will need to modify the feed holder to get your 7 dB back.

The WG of the Poty needs to be at the same place as the feed of the LNB was before. Exactly the same plane. At 10GHz its extrem sensitiv. 1cm mechanical mismatch is paid by loss of 3dB.

It takes me a while with my 60cm prime focus. Now it works wonderful at fully waterresistant in an grey water tube which you can buy at the local stores (Baunarkt) . See picture below if my minimal compact system for portable activity.

I have been doing some tests with 3D printed lens antennas provided by Malcolm, G0UHY. I was concentrating on the MER of the DATV beacon as a metric, but also recorded the strength of the CW beacon (noise floor is -115 dBm in 3.5kHz bandwidth). Here are the results:

Culprit

DATV Beacon MER

CW Beacon

Standard rocket Lens

10.7

-79

Short cone

9.5

-80

Long cone

10.1

-79

Cylinder

9.1

-81

No lens

9.6

-80

The dish is a 2.4m channel master offset. It has a F/D of 0.6. The poorer performing lenses may be better at lower F/D, but I only have one dish so can't test this.

I had Lense 1 or 2 with my poty kits, when I compared it with the Lenses from an Inverto and a Venton Rocket LNB (3rd from right on picture above) the lens from Venton Rocket give the best result, the Inverto was the second best and than the one was send with the poty kit.